The initial swing is one thing, but IP creep is another. A 10-20 psi initial swing is normal, but once it stops, there should be virtually no creep. If any is present, it should be less than 5 psi in about the first 5-10 seconds with none after that.
The Mk 20 with the mid production brass tipped piston is the major exception in terms of Scubapro first stages in that a 5 psi creep after initial lock up is normal. But they also tend to have problems locking up at all after a while in service so I almost always replace the piston with a new knife edged Mk 25 piston.
That behavior in a Mk 17 that is about 90 dives post service is however not outside the realm of the normal. A 5 to 10 psi creep over a few minutes is something I have found in my Mk 17's about the time they are due for service (usually around 100-130 dives in my case.) And the same pattern is common for many regs. The problem is that this type of slow and controlled creep usually precedes a much larger and faster creep that will not lock up at 145 ps and it can usually be expected to occur at some point in the not too distant future. Your Mk 17 is in effect telling you it is time to be serviced. You can ignore it, but the possible consequence will be a free flowing second stage and murphy's law dictates that will happen on dive one of day one of your next really expensive dive trip.
Age in and of itself does not mean anything as the seat is off the orifice when any first stage is depressuirized so they have have a very long post service shelf life. It is the actual use in diving that causes the wear through both the number of cycles and the contaminants (rust, aluminum oxide, sand, salt, carbon, etc.) that end up going through them. Modern lubricants are very good so most non-seat related wear comes from contaminants that enter the reg either in the breathing gas or in the rinse process and in a dirty reg, the seat wear can also be greatly accellerated by rust, salt or carbon particles that end up embedded in the seat. Clean gas iis one thing, but it is amazing how m8uch crud can end up in the average rental tank, so if rental tanks are involved it is not uncommon for a diver's reg to end up needing service sooner than those of a diver who owns his or her own very well maintained tanks.
In any event, with your Mk 17 part of the issue is probably normal seat wear but part of it is also probably also dirt/salt and lubrication issues and both will accellerate the wear that is occurring.
A Mk 2 that is not locking up after only 50 or so dives is a little odd as they are normally pretty bullet proof so I would tend to suspect contamination by rust carbon or salt on the seat that is interfering with a solid seal.
